svn commit: r350703 - in stable/12/sys/cam: . scsi

[Alexander Motin]

Author: mav
Date: Thu Aug  8 00:17:16 2019
New Revision: 350703
URL: https://svnweb.freebsd.org/changeset/base/350703

Log:
  MFC r340024 (by imp): Add comments explaining what hold/unhold do
  
  They act as a simple one-deep semaphore to keep open/close/probe from
  running at the same time to avoid races that creates.

Modified:
  stable/12/sys/cam/cam_periph.c
  stable/12/sys/cam/scsi/scsi_da.c
Directory Properties:
  stable/12/   (props changed)

Modified: stable/12/sys/cam/cam_periph.c
==============================================================================
--- stable/12/sys/cam/cam_periph.c	Wed Aug  7 21:20:25 2019	(r350702)
+++ stable/12/sys/cam/cam_periph.c	Thu Aug  8 00:17:16 2019	(r350703)
@@ -474,6 +474,12 @@ cam_periph_release(struct cam_periph *periph)
 	mtx_unlock(mtx);
 }
 
+/*
+ * hold/unhold act as mutual exclusion for sections of the code that
+ * need to sleep and want to make sure that other sections that
+ * will interfere are held off. This only protects exclusive sections
+ * from each other.
+ */
 int
 cam_periph_hold(struct cam_periph *periph, int priority)
 {

Modified: stable/12/sys/cam/scsi/scsi_da.c
==============================================================================
--- stable/12/sys/cam/scsi/scsi_da.c	Wed Aug  7 21:20:25 2019	(r350702)
+++ stable/12/sys/cam/scsi/scsi_da.c	Thu Aug  8 00:17:16 2019	(r350703)
@@ -2737,9 +2737,10 @@ daregister(struct cam_periph *periph, void *arg)
 	TASK_INIT(&softc->sysctl_task, 0, dasysctlinit, periph);
 
 	/*
-	 * Take an exclusive refcount on the periph while dastart is called
-	 * to finish the probe.  The reference will be dropped in dadone at
-	 * the end of probe.
+	 * Take an exclusive section lock qon the periph while dastart is called
+	 * to finish the probe.  The lock will be dropped in dadone at the end
+	 * of probe. This locks out daopen and daclose from racing with the
+	 * probe.
 	 *
 	 * XXX if cam_periph_hold returns an error, we don't hold a refcount.
 	 */